According to BBC Washington correspondent, Paul Reynolds, they are a compromise between the Americans, who want a full-scale naval blockade, and the French, who argue that this would be illegal without UN authorisation.

The search procedures are not yet comprehensive - for example, what happens if a ship defies patrols in the Adriatic has not been decided.

US troops and firepower are on their way to protect Apache helicopters in Albania

Serbian media reports said 26 missiles struck the city in two intense waves of bombing.

They were reported to have hit a factory, a railway shunting yard, a water works and electric power lines.

Local sources said a residential area had also been hit, with one civilian injured.

There were also strikes against the Kosovo capital, Pristina, with heavy damage to Slatina airport and the communications tower on Mount Goles among others.

The BBC correspondent says many of these places have bee hit time and time again, helping to confirm the widely held view among Yugoslav citizens that despite its denials, Nato is trying to make life as difficult as possible for the people of Serbia.

'Nato will prevail'

According to Nato, the oil embargo and other economics sanctions are part of an attempt to intensify pressure on the Yugoslav President Slobodan Milosevic.

A seventeen-point statement on Kosovo was issued at Nato's 50th anniversary summit in Washington emphasised that Nato will prevail over the Serbian leader,

Nato leaders said air strikes would continue until Yugoslavia met all their demands over Kosovo.

Reinforcements expected

As the war of words escalated, the US announced plans to send 15 tanks to Albania to help protect the newly arrived Apache helicopters.

The tanks will join 2,000 extra troops, and further supplies of rocket launchers and artillery being deployed in the region.

According to the Pentagon, the new fire power was requested by Nato's Supreme Commander, General Wesley Clark.

Serbia hampers Montenegro imports

Montenegro says Serbian forces are trying to prevent imports of medicines and other humanitarian aid entering the country through the port of Bar, on the Adriatic coast.

Although Montenegro forms part of the Federal Republic of Yugoslavia, the republic does not support President Milosevic, and supplies are not being passed on into Serbia.

But the Yugoslav army and navy maintain a strong presence around the port, and are able to police the embargo by threatening to confiscate any imports not authorised by Belgrade.